This is Omron quadrature incremental rotary encoder E6A2-CW5C with 200 P/R (pulse per revolution) of compact size (25mm external diameter). The encoder outputs gray code which you can interpret using a PLC,  microcontroller or Arduino and find out which direction the shaft is turning and by how much. 

This allows you to add feedback to motor control systems. Encoders of this kind are often used in medical devices, productions lines, CNC machines and dead reckoning navigation.

Features

• Resolution: 200 Pulse/Rotation
• Encoding Method: Incremental
• Input Voltage: 12 - 24VDC
• Maximum Rotating Speed: 5000rpm
• Allowable Radial Load: 10N
• Allowable Axial Load: 5N
• Cable Length: 50cm
• Shaft Diameter: 4mm
• External Diameter: 25mm

Datasheet for encoder E6A2-CW5C

Encoder manual and Arduino example code